In recent years, techniques are being studied for converting sustainable biomass into useful substances and using the same as an energy source or industrial raw material to replace petroleum resources. Ethanol and other useful substances produced by microbial fermentation using biomass as the raw material are promising as substitute raw materials from the standpoint of reducing petroleum consumption and controlling the increase of carbon dioxide in the atmosphere. Of the various kinds of biomass, herbaceous and woody plants and the like consisting primarily of lignocellulose may be useful as raw materials because they are not suitable as foodstuffs.
The principal sugars in lignocellulose are the glucose making up cellulose and the xylose making up hemicellulose. A saccharified composition containing primarily these simple sugars is obtained by chemical or enzymatic decomposition of lignocellulose. For the industrial manufacture of useful substances from lignocellulose, there is a need for microorganisms that efficiently utilize the sugars in such a saccharified composition and can ferment them productively and with high yield.
Saccharomyces cerevisiae and other yeasts with strong ethanol fermentation ability can generally utilize glucose, mannose and galactose, but not xylose. Thus, for efficient fermentation using lignocellulose as a raw material, these yeasts need to be modified to make them capable of utilizing xylose. The use of xylose isomerases (XI), isomerizing enzymes that convert xylose to xylulose, has been reported as a way for yeasts and the like to utilize xylose (Patent Literatures 1 and 2).
It has been reported that the activity of the following XI enzymes is sufficiently expressed in yeasts: XI from an anaerobic mold Piromyces sp. E2 (Patent Literature 1), XI from the anaerobic mold Cyllamyces aberensi, XI from the bacteria Bacteroides thetaiotaomicron (Patent Literature 2), and XI from the bacteria Clostridium phytofermentans (Non-patent Literature 1). XI genes from various organisms other than the above have also been introduced into yeasts, but have not been able to express their activity sufficiently (Non-patent Literatures 1, 2, 3 and 4). It has been found that xylose isomerases have common conserved regions (Non-patent Literature 5), and since XI enzymes that are actively expressed in yeasts and those that are not actively expressed all carry these conserved regions, the possession of a conserved region is not a sufficient condition for active expression of an XI in yeasts. The sequence characteristics necessary for active expression in yeasts are still entirely unknown.
There has also been very little research into the enzymological characteristics of xylose isomerases that function effectively in yeasts (Non-patent Literatures 1, 6 and 7).
Termites and other wood-degrading insects that use the woody component cellulose as an energy source break down cellulose with cellulose-degrading enzymes called cellulases. Cellulases from these insects are known to have extremely strong cellulose-degrading ability. The cellulases that act on cellulose in the termite gut are classified generally into two kinds: the termite's own cellulases and those belonging to symbiotic protists and other microorganisms in the termite's gut. Symbiotic protists living in the hindgut of lower termites play a principal role in cellulose decomposition, but these protists are difficult to culture, and past research has not advanced very far. At present, cellulases and genes from termite protists have been disclosed (Patent Literatures 3 and 4).